U.S. patent number 5,212,827 [Application Number 07/650,277] was granted by the patent office on 1993-05-18 for zero intermediate frequency noise blanker.
This patent grant is currently assigned to Motorola, Inc.. Invention is credited to Joseph P. Heck, William R. Meszko.
United States Patent |
5,212,827 |
Meszko , et al. |
May 18, 1993 |
Zero intermediate frequency noise blanker
Abstract
An essentially zero intermediate frequency receiver (100) for
recovering an information signal from a received signal (110),
which includes means for blanking noise signals which may otherwise
deteriorate performance, comprises a receiver (10) for recovering
the information signal and a noise blanker (28). The receiver (28)
comprises at least one conversion mixer (32B) for operating on the
received signal (110) to provide an essentially baseband signal
(125B), at least one delay filter (40B) coupled to the conversion
mixer (32B) for producing a delayed essentially baseband signal,
and at least one blanker switch (S1-S4) for operating on the
delayed essentially baseband signal to temporarily prevent recovery
of the information signal in response to a control signal (58). To
provide the control signal (58), the noise blanker (28) is coupled
to the receiver (10) for operating on either the essentially
baseband signal (125B) or the received signal (110) as a noise
blanker input signal.
Inventors: |
Meszko; William R. (Fort Worth,
TX), Heck; Joseph P. (Fort Lauderdale, FL) |
Assignee: |
Motorola, Inc. (Schaumburg,
IL)
|
Family
ID: |
24608221 |
Appl.
No.: |
07/650,277 |
Filed: |
February 4, 1991 |
Current U.S.
Class: |
455/219; 455/223;
455/225; 455/324 |
Current CPC
Class: |
H03G
3/345 (20130101); H04B 1/30 (20130101) |
Current International
Class: |
H04B
1/30 (20060101); H03G 3/34 (20060101); H04B
001/16 () |
Field of
Search: |
;455/194.1,200.1,219,220,222-225,303,311,324,338,212,213
;358/167,177,176 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Eisenzopf; Reinhard J.
Assistant Examiner: Pham; Chi H.
Attorney, Agent or Firm: Agon; Juliana
Claims
What is claimed is:
1. An essentially zero intermediate frequency receiver for
recovering an information signal from a received signal said
receiver comprising:
receiver means for recovering said information signal
comprising:
a first conversion means for operating on said received signal to
provide an essentially baseband inphase signal;
a second conversion means for operating on said received signal to
provide an essentially baseband quadrature signal;
a first delay means coupled to said first conversion means for
producing a delayed essentially baseband inphase signal;
a second delay means coupled to said second conversion means for
producing a delayed essentially baseband quadrature signal;
at least one blanker switch means for operating on said delayed
essentially baseband signals to temporarily prevent recovery of
said information signal in response to a switch control signal;
means for providing a gain control signal; and
noise blanking means for blanking noise signals which may otherwise
deteriorate performance, said noise blanking means coupled to said
receiver means for operating on either at least one of said
essentially baseband signals or said received signal as a noise
blanker input signal to provide said switch control signal;
said noise blanking means comprising;
input filtering means for filtering said noise blanker input signal
to provide a filtered signal;
amplifier means for amplifying said filtered signal, in response to
said gain control signal, to provide an amplified signal; and
means for generating said switch control signal from said amplified
signal.
2. The receiver of claim 1 wherein said delay means comprises an
R-C lowpass filter.
3. The receiver of claim 1 wherein said conversion means is a pair
of mixers connected in parallel.
4. The receiver of claim 1 wherein said input filtering means
comprises at least one bandpass filter for filtering said received
signal to provide said filtered signal.
5. The receiver of claim 1 wherein said input filtering means
comprises
first lowpass filtering means for filtering said at least one of
said essentially baseband signals to provide said filtered
signal.
6. The receiver of claim 5 wherein said first lowpass filtering
means filters said essentially baseband inphase signal to provide
said filtered signal.
7. The receiver of claim 5 wherein said means for generating said
control signal further comprises:
second lowpass filtering means for receiving said amplified signal
and providing a filtered output signal;
pulse detector means for operating on said filtered output signal
to provide an output pulse whenever the noise contents of said
filtered signal exceeds a predetermined threshold; and
pulse shaper means for receiving said output pulse and for
providing a desired output pulse.
8. An essentially zero intermediate frequency receiver for
recovering an information signal from a received signal said
receiver comprising:
means for splitting said received signal into a pair of input
signals;
receiver means for recovering said information signal
comprising:
at least one conversion means for operating on each one of said
pair of input signals to provide a pair of essentially baseband
signals in phase quadrature;
at least one lowpass delay means coupled to each one of said
conversion means for producing a pair of delayed essentially
baseband signals in phase quadrature; and
at least one blanker switch means for operating on said pair of
delayed essentially baseband signals to temporarily prevent
recovery of said information signal in response to a control
signal;
means for providing an automatic gain control (AGC) signal for
controlling the sensitivity of a noise blanking means; and
said noise blanking means for blanking noise signals which may
otherwise deteriorate performance, said noise blanking means
coupled to an output of at least one of said conversion means to
provide a noise blanker input signal for operating on at least one
of said essentially baseband signals to provide said control
signal;
said noise blanking means comprising:
input filtering means for filtering at least one of said
essentially baseband signals to provide a filtered signal;
amplifier means for amplifying said filtered signal, in response to
said gain control signal, to provide an amplified signal; and
means for generating said switch control signal from said amplified
signal.
9. The receiver of claim 8 wherein said input filtering means is
a
first lowpass filter for setting the bandwidth of said noise
blanking means to determine the amount of frequency spectrum that
said noise blanking means will monitor for noise.
10. The receiver of claim 9 wherein said control signal generating
means further comprises:
second lowpass filter for receiving said amplified signal and
providing a filtered output signal;
pulse detector means for operating on said filtered output signal
to provide an output pulse whenever the noise contents of said
filtered signal exceeds a predetermined threshold; and
pulse shaper means for receiving said output pulse and for
providing a desired output pulse.
11. The receiver of claim 10 further comprising:
AGC detecting means, coupled to an output of said second lowpass
filter for providing said AGC signal in a feedback loop comprising
said second lowpass filter, said AGC detecting means, and said
amplifier means.
12. An essentially zero intermediate frequency receiver for
recovering an information signal from a received signal, said
receiver comprising:
means for splitting said received signal into first, second, and
third input signals;
receiver means for recovering said information signal
comprising:
at least one conversion means for operating on each one of said
first and second input signals to provide a pair of essentially
baseband signals in phase quadrature;
at least one lowpass delay means coupled to each one of said
conversion means for producing a pair of delayed essentially
baseband signals in phase quadrature; and
at least one blanker switch means for operating on said pair of
delayed essentially baseband signals to temporarily prevent
recovery of said information signal in response to a control
signal; and
means for providing an automatic gain control (AGC) signal for
controlling the sensitivity of a noise blanking means; and
said noise blanking means coupled to an input of said conversion
means for operating on said third input signal to provide said
control signal;
said noise blanking means comprising:
input filtering means for filtering said third input signal to
provide a filtered signal;
amplifier means for amplifying said filtered signal, in response to
said AGC signal, to provide an amplified signal; and
means for generating said switch control signal from said amplified
signal.
13. The receiver of claim 12 wherein said input filtering means is
a
tuned bandpass filter tuned to a fixed frequency spectrum where
noise is expected.
14. The receiver of claim 13 wherein said control signal generating
means further comprises:
tuned bandpass filtering means for receiving said amplified signal
and providing a filtered output signal;
pulse detector means for operating on said filtered output signal
to provide an output pulse whenever the noise contents of said
filtered signal exceeds a predetermined threshold; and
pulse shaper means for receiving said output pulse and for
providing a desired output pulse.
15. The receiver of claim 14 wherein said means for providing an
automatic gain control (AGC) signal comprises a
received signal strength indicator for indicating the strength of
said received signal to provide said AGC signal.
Description
TECHNICAL FIELD
This invention relates generally to noise blankers and more
particularly to those communication devices that employ noise
blankers and essentially zero intermediate frequencies.
BACKGROUND
Those skilled in the art will appreciate the harsh operating
environment of communication devices such as mobile radios. The
major contributors to a severely noisy environment for the mobile
radio include engine noise, (both from the vehicle using the mobile
radio and surrounding vehicles), electrical interference from high
power lines, and atmospheric disturbances.
Some mobile radios have employed noise blankers to suppress or
eliminate these noise effects. The basic purpose of a noise blanker
is to detect the presence of impulse-type noise and momentarily
prevent the noise in the recovered signal from reaching the
intermediate frequency (IF). For the noise blanker to function
properly, it must detect the presence of noise and inhibit the
signal path in the main receiver before the noise gets to the point
where it is to be stopped. Historically, implementation of a noise
blanker in a mobile receiver was facilitated by the commensurate
bandwidth of the main receiver and the noise blanker (i.e. each
about 1 megahertz). Thus, the "race" condition was not a
significant problem. Since the bandwidths were practically the
same, the delay was effectively the same or could be compensated
for by small "lump element" filters.
Modern mobile radios however, have extremely broad bandwidths.
Since most mobile radios have frequency synthesizers that can
generate a wide variety of frequencies, mobile radios today use
broad bandwidth filters permitting the mobile radio user to operate
over a wide band of frequencies. Thus it is common for a receiver
to have bandwidth of 20 or 30 megahertz. However, this bandwidth
extension creates significant problems in the operation of the
noise blanker circuitry. Since the band width of the main receiver
may be twenty times the bandwidth of the noise blanker (thus making
the noise blanker delay 20 times that of the main receiver),
control pulses can not reach the blanker switch in time to prevent
the noise from entering the receiver IF. To compensate for a delay
of this magnitude, a "lump-element" filter cannot be used since the
current trend is toward radio size reduction. Hence, the size of
such a filter would be prohibitive.
A solution to the delay problem was achieved using a surface
acoustic wave (SAW) filter to afford both selectivity and time
delay in an appropriately sized filter. However, SAWs are expensive
commodities.
To further achieve miniaturization, microelectronic techniques are
desired in fabricating radios. Receivers producing substantially
low frequency intermediate frequency (IF) signals are known to be
easier to implement microelectronically for the intermediate stage.
Since this I.F. frequency may be substantially zero Hertz (i.e. DC
or baseband), the term zero I F (ZIF) is used in describing such an
IF signal or stage. "Direct conversion" receivers further utilizes
the ZIF advantage to eliminate a prior stage by converting an
incoming signal directly to baseband. With ZIF or direct
conversion, the necessary sharp selectivity is then achieved
through lowpass rather than bandpass filtering. Since low frequency
lowpass filters are readily fabricated in monolithic form, a much
greater degree of miniaturization can be achieved in proportion to
the amount of bandpass filters being converted into lowpass.
Thus a need exists to provide effective noise blanking while
contemporaneously providing broad receiver bandwidth and radio size
reduction.
SUMMARY OF THE INVENTION
Utilization of ZIF signals in the receiver provides some
advantages, namely, it eliminates the need for complex high
frequency bandpass IF filters, and facilitates integration of the
IF circuitry on an integrated circuit (IC) chip.
Accordingly, it is an advantage of the present invention to provide
noise blanking in a "zero-I.F." receiver.
Briefly, according to the invention, an essentially zero
intermediate frequency receiver for recovering an information
signal from a received signal, which includes means for blanking
noise signals which may otherwise deteriorate performance,
comprises a receiver for recovering the information signal and a
noise blanker. The receiver comprises at least one conversion mixer
for operating on the received signal to provide an essentially
baseband signal, at least one delay filter coupled to the
conversion mixer for producing a delayed essentially baseband
signal, and at least one blanker switch for operating on the
delayed essentially baseband signal to temporarily prevent recovery
of the information signal in response to a control signal. To
provide the control signal, the noise blanker is coupled to the
receiver for operating on either the essentially baseband signal or
the received signal as a noise blanker input signal.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a radio employing a noise blanker of
the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, a noise blanker 28 of the present invention is
included in a portion of an FM receiver 100 having a main receiver
10. Preferably, the circuits comprising this portion of the
receiver 100 utilize bipolar and metal oxide semiconductor (BIMOS)
technology for integrating the circuit on an IC chip. The receiver
100 may be used in radio communication units, such as mobile
two-way transceivers.
In the receiver 100, a received radio frequency (RF) signal or
intermediate frequency (IF) 110 is amplified by a preamplifier 12,
which produces an amplified signal 115. The input signal 115 from
the preamplifier 12 is supplied to each of two parallel,
substantially identical paths 32A-40A-S1-33A and
32B-40B-S2-33B.
Conventionally, elements 32A and 32B are down-conversion mixers
that translate the incoming signal 115 to essentially baseband. A
down-conversion frequency is supplied in quadrature to both mixers
32A and 32B using a phase shifter 36 or equivalent to provide two
signals in phase quadrature. The frequency of the LO signal 112 is
selected such that it is substantially equal to the frequency of
the received signal 110. In more detail, the phase-shifting circuit
36 receives the local oscillator (f.sub.DOWN) waveform 112 and
produces an inphase waveform (I) and a quadrature waveform (Q) in
response to the f.sub.DOWN waveform. The down mixers 32A-B convert
the signal from the RF to essentially baseband frequency.
Therefore, the pair of IF signals 125A and 125B have a
substantially low frequency and are modulated at the baseband
frequency. The outputs of mixers 32A and 32B are fed to two
identical low pass filters 33A and 33B which remove any received
spurious signals and limits the noise bandwidth of the receiver
100.
The respective outputs of these lowpass filters are coupled to a
demodulator 52. Subsequently, a modulating signal may be recovered
by any suitable demodulation technique at the demodulated output.
The demodulation technique may preferably comprise upmixing, by an
upmixer, the ZIF signal with a second high frequency local
oscillator, and applying the output of the upmixer to a well known
phase lock loop (PLL) or other type demodulator. In the preferred
embodiment of the invention, modulating signal recovery is achieved
by applying the output of each of the low pass filters 33A and 33B
to a pair of suitable up conversion mixers, which produces a pair
of upmixer signals in phase quadrature. The up mixers thus convert
the baseband signals up to a convenient frequency for further
processing and demodulation.
According to the present invention, the conventional zero
intermediate frequency (IF) receiver has been modified by adding
delay low pass filters 40A and 40B at the output of the downmixers
32A and 32B, series switches S1-S2 at the inputs of the baseband
bandpass or lowpass filters 33A and 33B, and shunt switches S3 and
S4 from the inputs of the lowpass filters 33A and 33B to analog
ground. In addition, the noise blanker 28 controls the selective
opening and closing of the switches S1-S4 in the main receiver
10.
After down mixing in the down mixers 32A and 32B, the pair of
substantially zero baseband signals 125A and 125B are coupled to
the pair of delay elements 40A and 40B. The pair of delay elements
40A and 40B may be implemented as a simple lowpass filter using a
distributed RC delay line or with discrete components (resistors
and capacitors). The lowpass filters 40A and 40B time delay the
substantially zero IF signal for approximately 3 micro seconds and
thus provide the major amount of time delay in the main receiver
10.
The noise blanking switches S1-S4 provide the means by which the
received signal is interrupted and thus prevented from entering the
pair of lowpass filters 33A and 33B. The blanker switches S1 thru
S4 may be implemented using any suitable technology and may be, for
example, one or more field effect transistors (FET's) configured
either in series and/or in shunt (to the received signal path) to
provide the required attenuation. The blanker switches S1-S4 are
positioned between the delayed filter 40A and 40B and the main
selectivity (the lowpass filters 33A and 33B) so that the main
receiver 10 may "blank" after the downmixers 32A and 32B.
Normally, the blanker switches S1-S2 are "closed" and S3-S4 are
"opened" to couple the output of the downmixers 32A and 32B to the
input of the pair of the lowpass filters 33A and 33B to allow the
received signal to be processed by the demodulator 52 and
subsequent circuitry.
Thus, when the noise blanker 28 determines that a noise condition
exists, the blanker switches S1-S2 are momentarily "opened" (by
asserting a control input 58) to prevent the received signal from
entering the lowpass filters 33A and 33B and being demodulated by
the demodulator 52. In addition, the shunt switches S3 and S4
connect the delay filters' outputs to analog ground when noise is
present to prevent glitches. In this manner, a long recovery time
is prevented in the delay filter output circuits (which act as a
current sink) which may take place if the delay filters' outputs
are allowed to float to their maximum or minimum voltage levels.
The "open" duration is appropriately set to prevent the recovered
signal containing the noise from entering the lowpass filters 33A
and 33B, after which the blanker switches S1-S2 "close" and the
blanker switches S3-S4 revert to an "open" position permitting
normal operation.
To provide the control signal 58 to control the switches S1-S4, the
noise blanker 28 including filters 60 and 70 is coupled to the main
receiver 10 for operating on either the essentially baseband signal
125A or 125B or the received signal 115 as a noise blanker input
signal 128. The filter 60 sets the bandwidth of the noise blanker
28 and determines the amount of frequency spectrum that the noise
blanker 28 will monitor for noise. Depending on how the noise
blanker 28 is connected to the main receiver 10 to determine what
the noise blanker input signal 128 is, the filters 60 and 70 are
either bandpass or lowpass filters. The filtering is greatly
simplified from a bandpass filter centered at the noise blanker RF
frequency to a bandpass filter centered at DC which becomes a
lowpass filter.
In a fixed RF channel embodiment, independent of the desired RF
frequency, the noise blanker 28 accepts the received signal 115 at
an RF bandpass filter 60 tuned to a fixed RF channel where noise is
expected. Since the bandwidth of the main receiver 10 is broad
there may be several mobile radio users transmitting in the
allotted spectrum. Thus the tuned RF bandpass filter 60 of the
noise blanker 28 must be set or tuned to monitor a portion of the
frequency band that is not being used by other carriers or
information signals since they may be interpreted as noise and the
main receiver 10 will be inhibited. The bandpass filter may be
implemented by any topology that facilitates tuning and may be for
example, a 3 pole-coupled resonator filter having a 1 megahertz
bandwidth or suitable equivalent.
On the other hand, in the preferred embodiment for easier
microelectronic implementation, the noise blanker 28 accepts one of
the essentially baseband signals 125A or 125B or a weighted sum of
each at the more desired lowpass filter 60. In this embodiment, the
noise blanker 28 has an RF channel centered at the desired receive
baseband frequency since the I and Q signals are always centered at
baseband. Hence, the filter 60 may be implemented easily as a
lowpass filter having a bandwidth of approximately 0.5 megahertz
using resistors and capacitors, as opposed to high loaded Q band
pass filters or SAW delay lines. As the IF frequency drops and
approaches zero, this embodiment is preferred to enable usage of
more lowpass filters.
With either embodiments, the band-limited noise signal is then
applied to an automatic gain controllable (AGC) amplifier 64 which
accepts an AGC input signal at terminal 68. The AGC signal applied
at port 68 of the amplifier 64 increases or decreases the gain of
the amplifier 64 in the well known AGC operation.
The now appropriately amplified noise signal is applied to a tuned
RF bandpass filter 70 in the fixed RF channel embodiment or a
simple lowpass filter 70 in the preferred embodiment to again
band-limit the signal which is then coupled to a pulse detector 72.
The pulse detector 72 monitors the amplified band-limited signal
and compares it to a predetermined threshold to determine when
noise spikes (or pulses) are present. When the noise peaks exceed
the predetermined threshold the pulse detectors 72 outputs a pulse
indicating that excessive noise is present. The pulse output from
the pulse detector 72 is amplified in an optional separate pulse
amplifier 74 (or incorporated in a pulse shaper 76) which provides
sufficient gain to the pulse to trigger a pulse shaper 76.
The pulse shaper 76 accepts the amplified "trigger" pulse and first
generates a substantially rectangular pulse which is then shaped
into a trapezoidal shape or any other desirable shapes have sloped
rising and falling edges and having a predetermined pulse duration.
The duration of the pulse or the control signal 58 generated by the
pulse shaper 76, is set to allow sufficient time for the blanker
switches S1-S4 to reach and maintain maximum attenuation, thus
preventing the noise signal, being delayed by the pair of delayed
filters 40A and 40B, from entering the pair of lowpass filters 33A
and 33B. Accordingly, the duration of the pulse generated by the
pulse shaper 76 may be set to an appropriate duration to allow the
blanker switches S1-S4 to reach full attenuation and remain "open"
until the noise signal has sufficient time to pass through the
delay filters 40A and 40B taking into account the varying
parameters.
A rate shutoff circuit 86 is shown as an optional feature for the
noise blanker 28. As is known, rate shutoff circuit measure the
repetition rate of detected noise pulses without regard to their
amplitude. If the rate exceeds a predetermined value, the circuit
86 will disconnect the blanking function from the essentially Zero
IF signal, since if the repetition is too high, no signal will be
recovered anyway since "blanking" will be continuous.
As previously mentioned, the amplifier 64, and thus the noise
blanker 28, is controlled by the AGC signal. Generally, an AGC
signal is commonly used in AM receivers as a control for
amplifiers. Basically, the goal of the AGC circuit is to reduce the
gain of the blanker RF channel when the desired signal increases,
thereby desensitizing the blanker 28 and increasing the minimum
noise pulse amplitude required to initiate blanking. As the desired
signal level increases, the smaller noise pulses no longer create
objectionable interference, whereas blanking would create
interference. Accordingly, an AGC circuit including an AGC RF
amplifier and detector 78 controls the gain of an AGC amplifier 64
and reduces the gain to reduce the sensitivity of the noise blanker
28 when the desired received signal exceeds the threshold
level.
In the preferred embodiment of using a lowpass filter 60 in the
noise blanker 28 to feed in the essentially baseband signal 125A or
125B, both noise and the desired signal is received since the
blanker's RF channel (60) is centered at the desired receive
(essentially baseband) frequency. Accordingly, the noise signal and
the desired signal received and filtered by the lowpass filter 70
is utilized as a feed back signal 145 to control the AGC amplifier
and detector 78. On the other hand, in the alternate embodiment of
the fixed RF channel being tuned for expected noise in the noise
blanker 28, the desired signal is not present in the fixed RF
channel since the tuned RF filters are intentionally tuned to
eliminate the desired signal. Therefore, another source of AGC
control is needed. Hence, in the main radio receiver 10 (maybe from
the demodulator 52), a received signal strength indicator (RSSI)
signal from the RSSI 160 may be utilized to indicate the strength
of the signal (including noise) received. Thus, the AGC signal can
be developed from the RSSI 160 where the RSSI signal output from
the RSSI 160 is a DC voltage which varies proportionally to the
signal strength of the received signal including noise. Coupled
from the RSSI 160, the RSSI signal is applied to the AGC port 68 of
the amplifier 60 to control the gain in the well known AGC
operation.
In summary, the noise blanking circuitry can be greatly simplified
for a receiver with a zero IF because the IF filtering is done at
audio frequencies. In this case, filtering normally done with
narrow-band RF tuned bandpass circuits or the equivalent can be
replaced with lowpass filters. The filtering is greatly simplified
from a bandpass filter centered at the noise blanker RF frequency
to a lowpass filter centered at DC. The lowpass filter can be
implemented using either simple resistors and capacitors (RC)
inductors and capacitors (LC), or active integrated filters. If
lossiness is not a big problem, RC's are probably preferable since
they are easier to integrate.
* * * * *